Density-functional Perturbation Theory

Praha, September 3, 2019 1

Density-functional

Perturbation Theory

X. Gonze Université catholique de Louvain and Skolkovo Institute of Technology


Properties of solids from DFT

Computation of ... interatomic distances, angles, total energies electronic charge densities, electronic energies

A basis for the computation of ...

chemical reactions electronic transport vibrational properties thermal capacity dielectric response optical response superconductivity surface properties spectroscopic responses ...


“Classic” References : S. Baroni, P. Giannozzi and A. Testa, Phys. Rev. Lett. 58, 1861 (1987) X. Gonze & J.-P. Vigneron, Phys. Rev. B 39, 13120 (1989) X. Gonze, Phys. Rev. A 52 , 1096 (1995) S. de Gironcoli, Phys. Rev. B 51 , 6773 (1995) X. Gonze, Phys. Rev. B. 55 , 10337 (1997) X. Gonze & C. Lee, Phys. Rev. B. 55 , 10355 (1997) S. Baroni, et al., Rev. Mod. Phys. 73, 515 (2001)

Overview1.  A brief reminder : Density Functional Theory 2.  Material properties from total energy derivatives : phonons 3.  Perturbations (adiabatic) 4.  Perturbation Theory : « ordinary » quantum mechanics 5.  Density-Functional Perturbation Theory (DFPT) 6.  Phonon band structures from DFPT 7.  Thermodynamic properties from DFPT 8.  Phonons : LDA ? GGA ? 9.  Phonons in weakly bonded systems


VKS(r) =Vext (r)+n(r')r'- r∫ dr'+ δExc n[ ]

δn(r)

The Kohn-Sham orbitals and eigenvalues Non-interacting electrons in the Kohn-Sham potential :

Hartree potential Exchange-correlation potential

Density

To be solved self-consistently ! Note. At self-consistency, supposing XC functional to be exact : -  the KS density = the exact density, -  the KS electronic energy = the exact electronic energy -  but KS wavefunctions and eigenenergies correspond to a fictitious

set of independent electrons, so they do not correspond to any exact quantity.

−12∇2 +VKS(r)

⎛⎝⎜

⎞⎠⎟ψ i (r) = εiψ i (r)

n(r) = ψ i*(r)ψ i (r)

i∑


Minimum principle for the energy

Variational principle for non-interacting electrons : solution of KS self-consistent system of equations is equivalent to the minimisation of

under constraints of orthonormalization for the occupied orbitals.

EKS ψ i{ }⎡⎣ ⎤⎦ = ψ i −12∇2 ψ i

i∑ + Vext (r)n(r)∫ dr + 1

2n(r1)n(r2 )r1 - r2

∫ dr1dr2 + Exc n[ ]

ψ i ψ j = δ ij


The XC energy

To be approximated ! Exact result : the XC energy can be expressed as Exc n[ ] = n(r1)ε xc (r1;n)∫ dr1

Local density approximation (LDA) : -  local XC energy per particle only depends on local density -  and is equal to the local XC energy per particle of an

homogeneous electron gas of same density (« jellium ») εxc

LDA(r1; n ) = εxchom( n(r1) )

Generalized gradient approximations (GGA)

In this talk, GGA = « PBE » Perdew, Burke and Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)

ExcGGA n[ ] = n(r1)εxc

GGA(n(r1), ∇n(r1) )∫ dr1

ExcLDA n[ ] = n(r1)εxc

LDA(n(r1))∫ dr1


Material properties from total energy derivatives :

phonons


Changing atomic positions

E BO

Born-Oppenheimer approximation …


Phonon frequencies from force constants

� Fourier Transform (using translational invariance) :��

Computation of phonon frequencies and eigenvectors = solution of generalized eigenvalue problem

!Ckα,k'α '

"q( ) = Ckα,k'α 'a'∑ (0,a') ei"q.

"Ra'

!Ckα,k'α '

k'α '∑

"q( ).um"q (k'α ') = M k . ωm"q2 . um"q (kα)

phonon displacement pattern masses square of

phonon frequencies

How to get second derivatives of the energy ? Density Functional Perturbation Theory...

Cκα,κ'α ' a,a '( ) = ∂2EBO∂Rκα

a ∂Rκ'α 'a'Matrix of interatomic force constants :�

��


Phonons : exp vs theory

Diamond

Zircon

XG, G.-M. Rignanese and R. Caracas. Zeit. Kristall. 220, 458-472 (2005)

Rignanese, XG and Pasquarello. Phys. Rev. B 63, 104305 (2001)

Phonons at zone center


In addition of being able to compute derivatives of BO energy :

Treating phonons of different wavelengths ? (Not only periodic ones)

Treating electric field ? Electric field => linear potential, incompatible with periodicity

Even for phonons at zero wavevector (Gamma), treating LO-TO splitting (longitudinal optic – transverse optic)

Challenges for periodic materials ?


Perturbations (adiabatic)


Why perturbations ? Many physical properties = derivatives of total energy (or suitable thermodynamic potential) with respect to perturbations. Consider :

• atomic displacements (phonons) • dilatation/contraction of primitive cell • homogeneous external field (electric field, magnetic field ...)

Derivatives of total energy (electronic part + nuclei-nuclei interaction) : 1st order derivatives : forces, stresses, dipole moment ... 2nd order derivatives : dynamical matrix, elastic constants, dielectric susceptibility

atomic polar tensors or Born effective charge tensors piezoelectricity, internal strains ...

3rd order derivatives : non-linear dielectric susceptibility, Raman susceptibilities electro-optic effect, phonon - phonon interaction, Grüneisen parameters, ...

Further properties obtained by integration over phononic degrees of freedom : entropy, thermal expansion, phonon-limited thermal conductivity ...


Perturbations

* Variation of energy and density around fixed potential �� * Perturbations (assumed known through all orders)

i.e. : to investigate phonons, parameter of perturbation governs linearly nuclei displacement, but change of potential is non-linear in this parameter.

Eel λ( ) = ψα λ( ) T+Vext λ( ) ψα λ( ) + EHxc ρ λ( )⎡⎣ ⎤⎦α,occ∑

ρ(!r;λ) = ψα∗ (!r;λ) ψα (

!r;λ)α,occ∑

Vext λ( )= Vext(0) + λVext

(1) + λ2Vext(2) + ...

ΔVph (!r ) = Vκ (

κ: nuclei+cell∑

!r - (!Rκ(0)+!uκ )) - Vκ (

!r - !Rκ(0) )

!uκ = λ !eκ cos(!q . !Rκ(0) )

small parameter

‘polarisation’ of the phonon

phonon wavevector


How to get energy derivatives ?

* Finite Differences�

Compare and ��

‘Direct’ Approach (Frozen phonons ... Supercells …) [Note problem with commensurability] �

�* Hellman - Feynman theorem (for E(1)) Due to variational character : �

In order to get E(1) we do not need (1)

E ψ; Vext{ }

∂E∂ψ

= 0

dEdλ

= ∂E∂Vext

∂Vext

∂λ + ∂E

∂ψ . ∂ψ

∂λ = ∂E

∂VextVext

(1)

=

0 (1)

E' ψ '; V'ext{ }

ψψ

E = E(0) + λE(1) + λ2E(2) + ... ψ = ψ (0) + λψ (1) + λ2ψ (2) + ...


*

*

Hypothesis : we know �

through all orders, as well as Should calculate :

General framework of perturbation theory A λ( )= A(0) + λA(1) + λ2A(2) + λ3A(3)...

E ψ; Vext{ }Vext λ( )= Vext

(0) + λVext(1) + λ2Vext

(2) + ...

(0) , , E(0)

E(1) , E(2) , E(3)...

ψα(1) , ψα

(2) , ψα(3) ...

εα(1) , εα

(2) , εα(3) ...

ρα(1) , ρα

(2) , ρα(3) ...

2nd order derivatives of BO energy : dynamical matrix, dielectric susceptibility, elastic constants, …

ψ ρα(0)

will be needed for T-dependence of electronic structure


Ordinary quantum mechanics


Perturbation theory for ordinary quantum mechanics

Hamiltonian supposed known through all orders

H = H (0)+ λH (1)+ λ2H (2)+... = λnH (n)

n∑

(H - εα ) ψα = 0 (Schrödinger equation)

ψα ψα = 1 (normalisation condition)

ψα H - εα ψα = 0

or εα = ψα H ψα (expectation value)


Suppose

with

One expands the Schrödinger equation:

H(λ) ψ n (λ) = εn ψ n (λ) valid for all λ

Perturbation expansion of the Schrödinger Eq.

H(0) ψ n(0) + λ H(1) ψ n

(0) + H(0) ψ n(1)( ) + λ2 H(1) ψ n

(1) + H(0) ψ n(2)( ) + ...

= εn(0) ψ n

(0) + λ εn(1) ψ n

(0) + εn(0) ψ n

(1)( )+ λ2 εn(2) ψ n

(0) + εn(1) ψ n

(1) + εn(0) ψ n

(2)( )+ ...

H(λ) = H(0) + λ H(1)

ψ n (λ) = ψ n(0) + λψ n

(1) + λ2ψ n(2) + ...

εn (λ) = εn(0) + λ εn

(1) + λ2εn(2) + ...


If = 0, one gets no surprise … Derivative with respect to , then = 0 (=first order of perturbation)

=> 2 derivatives with respect to , then = 0 (=second order of perturbation)

=>

H(0) ψ n(0) + λ H(1) ψ n

(0) + H(0) ψ n(1)( ) + λ2 H(1) ψ n

(1) + H(0) ψ n(2)( ) + ...

= εn(0) ψ n

(0) + λ εn(1) ψ n

(0) + εn(0) ψ n

(1)( )+ λ2 εn(2) ψ n

(0) + εn(1) ψ n

(1) + εn(0) ψ n

(2)( )+ ...

H(0) ψ n(0) = εn

(0) ψ n(0)

H(1) ψ n(0) + H(0) ψ n

(1) = εn(1) ψ n

(0) + εn(0) ψ n

(1)

H(1) ψ n(1) + H(0) ψ n

(2) = εn(2) ψ n

(0) + εn(1) ψ n

(1) + εn(0) ψ n

(2)

Perturbation expansion of the Schrödinger Eq.

λ

λ λ

λ λ


with Same technique than for Schrödinger equation, one deduces :

∀λ : ψ n (λ) ψ n (λ) = 1If

ψ n(0) ψ n

(0) = 1

ψ n(1) ψ n

(0) + ψ n(0) ψ n

(1) = 0

ψ n(2) ψ n

(0) + ψ n(1) ψ n

(1) + ψ n(0) ψ n

(2) = 0

ψ n (λ) = ψ n(0) + λψ n

(1) + λ2ψ n(2) + ...

Perturbation expansion of the normalisation

no surprise …


H(1) ψ n(0) + H(0) ψ n

(1) = εn(1) ψ n

(0) + εn(0) ψ n

(1)

Hellmann & Feynman theorem : Start from first-order Schrödinger equation

Premultiply by

So : = Hellmann & Feynman theorem

•  and supposed known •  not needed •  = expectation of the Hamiltonian for the non-perturbed wavef.

ψ n(0) εn

(0)

ψ n(0)

ψ n(0) H(1) ψ n

(0) + ψ n(0) H(0) ψ n

(1) = εn(1) ψ n

(0) ψ n(0) + εn

(0) ψ n(0) ψ n

(1)

= = 1

εn(1) = ψ n

(0) H(1) ψ n(0)

ψ n(0) H(1)

ψ n(1)

ψ n(0) H(1) ψ n

(0)

εn(1) OK !

εn(1)


Start from second-order Schrödinger equation

Premultiply by

Second-order derivative of total energy

ψ n(0)

H(1) ψ n(1) + H(0) ψ n

(2) = εn(2) ψ n

(0) + εn(1) ψ n

(1) + εn(0) ψ n

(2)

εα(2) = ψα

(0) H (1)- εα(1) ψα

(1) or εα(2) = ψα

(1) H (1)- εα(1) ψα

(0)

Both can be combined :

εα(2) = 1

2 ψα

(0) H (1)- εα(1) ψα

(1) + ψα(1) H (1)- εα

(1) ψα(0)( )

and, using ψn(1) ψn

(0) + ψn(0) ψn

(1) = 0

= 12

ψα(0) H (1) ψα

(1) + ψα(1) H (1) ψα

(0)( ) No knowledge of is needed, but needs ! How to get it ? ψα

(2) ψα(1)

εα(2)


In search of Again first-order Schrödinger equation :

Terms containing are gathered :

Equivalence with matrix equation (systeme of linear equations)

usually solved by if exist.

ψ n(1)

A . x = y

H(1) ψ n(0) + H(0) ψ n

(1) = εn(1) ψ n

(0) + εn(0) ψ n

(1)

known known

H(0) - εn(0)( ) ψ n

(1) = - H(1) - εn(1)( ) ψ n

(0) (called Sternheimer equation)

x = A-1 y A-1

ψ n(1)


Variational Principle for the lowest (Hylleraas principle) ε(2)=min

ψ(1)ψ (1) H (1) ψ (0) + ψ (1) H (0) - ε(0) ψ (1) + ψ (0) H (2) ψ (0) + ψ (0) H (1) ψ (1){ }

with the following constraint on : �Allows to recover Sternheimer’s equation : + Lagrange multiplier

=>

Equivalence of : * Minimization of

* Sternheimer equation * also … sum over states … Green’s function …

ψ (0) ψ (1) + ψ (1) ψ (0) = 0

δδψ (1) [ ... ] = 0

(H (0)- ε(0) ) ψ (1) + (H (1)- ε(1) ) ψ (0) = 0

ψ n(1)

εn(2)

εα(2)


Computation of (I) εα(3)

Starting from

Premultiply by gives

(H (0)- εα(0) ) ψα

(3) + (H (1)- εα(1) ) ψα

(2) + (H (2)- εα(2) ) ψα

(1) + (H (3)- εα(3) ) ψα

(0) = 0

ψα(0)

εα(3) = ψα

(0) H (3) ψα(0)

+ ψα(0) H (2)- εα

(2) ψα(1)

+ ψα(0) H (1)- εα

(1) ψα(2)

! ψα(2) is needed in this formula


The computation of (II) εα(3)

However, perturbation expansion of at third order gives:

�The sum of terms in a row or in a column vanishes ! (Exercice !) Suppress 2 last columns and 2 last rows, rearrange the equation, and get:

0 = ψα H- εα ψα

0 = ψα(0) H (3) - εα

(3) ψα(0) + ψα

(1) H (2) - εα(2) ψα

(0) + ψα(2) H (1) - εα

(1) ψα(0) + ψα

(3) H (0) - εα(0) ψα

(0)

+ ψα(0) H (2) - εα

(2) ψα(1) + ψα

(1) H (1) - εα(1) ψα

(1) + ψα(2) H (0) - εα

(0) ψα(1)

+ ψα(0) H (1) - εα

(1) ψα(2) + ψα

(1) H (0) - εα(0) ψα

(2)

+ ψα(0) H (0) - εα

(0) ψα(3)

εα(3) = ψα

(0) H (3) ψα(0) + ψα

(1) H (2) ψα(0)

+ ψα(0) H (2) ψα

(1) + ψα(1) H (1)-εα

(1) ψα(1)

[ We have used and ]ψα(0) ψα

(0) = 1 ψα(0) ψα

(1) + ψα(1) ψα

(0) = 0

! ψα(2) is not needed in this formula


Generalisation: Density-functional perturbation theory

(DFPT)


Density functional perturbation theory

Without going into the formulas, there exist expressions :

E(0) ψα(0){ } ψα

(0)

E(1) ψα(0){ }

E(2) ψα(0);ψα

(1){ } ψα(1)

E(3) ψα(0);ψα

(1){ }E(4) ψα

(0);ψα(1);ψα

(2){ } ψα(2)

E(5) ψα(0);ψα

(1);ψα(2){ }

variational with respect to



knowledge of allows one to obtain

+ knowledge of allows one to obtain

knowledge of allows one to obtain ψα(0);ψα

(1);ψα(2){ } n(2),H(2), εα

(2)ψα(0);ψα

(1){ }

ψα(0){ } n(0),H(0), εα

(0)

n(1),H(1), εα(1)

Need unlike in ordinary QM ψα(2)


Basic equations in DFT

Solve self-consistently Kohn-Sham equation

H ψn = εn ψn

n(!r ) = ψn

* (!r )ψn (!r )

n

occ∑

H =T+ V+VHxc[n]

What is ? V

Eel ψ{ } = ψn T+ V ψnn

occ∑ +EHxc[n]

V(!r) = - Zκ!r-!Rκaaκ

∑

or minimize

ψ n (r)

n(r)

H

δmn = ψm ψn for m,n ∈occupied set


Basic equations in DFPT

Solve self-consistently Sternheimer equation

(H (0)-εn(0) ) ψn

(1) = - (H (1)-εn(1) ) ψn

(0)

n(1)(!r ) = ψn

(1)*(!r )ψn(0)(!r )+ψn

(0)*(!r )ψn(1)(!r )

n

occ∑

H (1) = V (1)+ δ2EHxcδρ(r)δρ(r') n

(1)(r')dr'∫

εn(1) = ψn

(0) H (1) ψn(0)

What is , ? V (1)

Eel(2) ψ (1);ψ (0){ } = ψn

(1) H (0)-εn(0) ψn

(1)

n

occ∑ + ψn

(1) V (1) ψn(0)

+ ψn(0) V (1) ψn

(1) + ψn(0) V (2) ψn

(0)

+ 12

δ2EHxcδρ(!r )δρ(!r') n

(1)(!r ) n(1)(!r')∫∫ d!r d!r'

or minimize

ψ n(1) (r)

n(1)(r)

H (1)

V (2)

0 = ψm(0) ψn

(1) for m ∈occupied set


The potential and its 1st derivative

V (0)(!r) = - Zκ!r-!Rκaaκ

∑

V (1)(!r) = ∂V(!r)∂Rκ ,α

a = Zκ!r-!Rκa 2 .

∂ !r-!Rκa

∂uκ ,αa = - Zκ

!r-!Rκa 3 . !r-

!Rκa( )α

Derivative with respect to Rκαa

Collective displacement with wavevector !q

Generalisation to pseudopotentials can be worked out ...

V!q,κ ,α

(1) (!r) = ei!q!Ra

a∑

∂V(!r)∂Rκ ,α

a


Factorization of the phase

Suppose unperturbed system periodic If perturbation characterized by a wavevector :

all responses, at linear order, will be characterized by a wavevector : Now, define related periodic quantities In equations of DFPT, only these periodic quantities appear: phases and can be factorized Treatment of perturbations incommensurate with unperturbed system periodicity is thus mapped onto the original periodic system.

V(0)(!r+

!Ra ) = V (0)(!r )

V(1)(!r+

!Ra ) = ei!q.

!Ra V (1)(!r )

n(1)(!r+

!Ra ) = ei!q.

!Ra n(1)(!r )

n(1)(!r ) = e-i!q !r n(1)(!r )

e-i!q.!r

ψm,

!k,!q

(1) (!r+!Ra ) = ei(

!k+ !q)

!Ra ψm,

!k,!q

(1) (!r )

um,!k,!q

(1) (!r ) = (NΩ0 )1/2 e-i(

!k+ !q)!r ψm,

!k,!q

(1) (!r )

e-i(!k+!q)!r


Computing mixed derivatives How to get E j1 j2 from ?

ψα(0) , ψα

j1 , ψαj2

Eel(2) ψ (1);ψ (0){ } = ψn

(1) H (0)-εn(0) ψn

(1)

n

occ∑ + ψn

(1) V (1) ψn(0)

+ ψn(0) V (1) ψn

(1) + ψn(0) V (2) ψn

(0)

+ 12

δ2EHxcδρ(!r )δρ(!r') n

(1)(!r ) n(1)(!r')∫∫ d!r d!r'

Generalization to

!Eelj1 j2 ψ j1 , ψ j2 ;ψ (0){ } = ψn

j1 H (0)-εn(0) ψn

j2

n

occ∑ + ψn

j1 V j2 ψn(0)

+ ψn(0) V j1 ψn

j2 + ψn(0) V j1 j2 ψn

(0)

+ 12

δ2EHxcδρ("r )δρ("r') n

j1 ("r ) n j2 ("r')∫∫ d"r d"r'

Eel

j1 j2 = 12!Eel

j1 j2 + !Eelj2 j1( )

with

being a stationary expression, leading to the non-stationary expression Eel

j1 j2 ψ j1;ψ (0){ } = ψnj1 V j2 ψn

(0)

n

occ∑ + ψn

(0) V j1 j2 ψn(0)

Independent of ψ j2


Order of calculations in DFPT (1) Ground-state calculation

(2) Do for each perturbation j1

use using minimization of second-order energy or Sternheimer equation Enddo

(3) Do for each { j1, j2}

get E j1 j2 from

Enddo

(4) Post-processing : from ‘bare’ E j1 j2 to physical properties

V (0) → ψn(0) , n(0)

ψn(0) , n(0)

V j1 → ψnj1 , n j1

ψn(0) , ψn

j1 , ψnj2


Phonon band structures from DFPT


From DFPT : straightforward, although lengthy (self-consistent calculation) to compute, for one wavevector :

Full band structure needs values for many wavevectors ...

)(~', qC kk!

βα

XG, J.-C.Charlier, D.C.Allan, M.P.Teter, Phys. Rev. B 50, 13055 (1994)

SiO2 alpha-quartz

Phonon band structure


If IFCs were available, dynamical matrices could be obtained easily for any number of wavevectors

IFCs are generated by

= Fourier interpolation of dynamical matrices.

Cκα ,κ 'β (0,b) =(2π )3

Ω0

!Cκα ,κ 'β (!q)e−i

!q⋅!Rb d!q

BZ∫

!Cκα ,κ 'β (!q) = Cκα ,κ 'β (0,b)e

i!q⋅!Rb

b∑

Fourier Interpolation


Cκα ,κ 'β (0,b) =(2π )3

Ω0

!Cκα ,κ 'β (!q)e−i

!q⋅!Rb d!q

BZ∫

Key of the interpolation : replace the integral

by summation on a few wavevectors (=“q-points”).

xq

Yq

Fourier

Grid of (l,m,n) points IFC’s in box of (l,m,n) periodic cells

Numerical Fourier Interpolation


Fourier interpolation : Silicon

Real space IFC’s calculated with 10 q-points Real space IFC’s calculated with 18 q-points


Interatomic force constants for silicon

NN

= total

XG, Adv. in Quantum Chemistry 33, 225 (1999)

IFC’s are short range, i.e. falling to zero quickly after the nearest-neighbors (NN).


Interatomic force constants for silica quartz

NN

NN

= total

= dipole-dipole = short - range

Quartz 3 Si 6 O

Si

O

XG, Adv. in Quantum Chemistry 33, 225 (1999)

Long-ranged interatomic forces !


Understanding the long-range behaviour When a ion with charge Z is displaced from its equilibrium position, a dipolar electric field is created. Its effect on other ions is described by a dipole - dipole interaction appearing in IFC’s.

Suppose : homogeneous material with isotropic dielectric tensor , ions with charges Zk and Zk’ , then

εδαβ

Long range decay of the IFC’s : 1/d3

+

+

-

-

⎟⎟⎠

⎞⎜⎜⎝

⎛−= 53

'', 3),0(

ddd

dZZbC kk

kkβααβ

βαδ

ε

kkaa

kk Rrrd ττ −+=−= ''0


!Cκα ,κ 'βna ("q→ 0) = 4πe

2

Ω0

Zκ ,αγ∗ qγ

γ∑ Zκ ',βv

∗ qvv∑

qγ εγ v∞ qv

γ ,v∑

electronic dielectric tensor (electronic contribution to the screening of the charges)

Born effective charge tensor for atom

Effect of the long-range interaction The dynamical matrix exhibit a non-analytical (na) behavior, mediated by the long-wavelength electric field

(Proportionality coefficient between polarisation and displacement, also between force and electric field)

κ

Both can be linked to a second derivative of total energy

εγ v∞ = δγν + 4π

∂Pγ∂εν

Zκ ,αβ∗ =Ω0

∂Pα∂uκ ,β δ

!E=0

=∂Fκβ∂εβ


Interpolation Scheme Use Abinit to calculate on a few Q-point.

Subtract the dipole-dipole coupling

Calculate and

Use the real space IFC’s to interpolate at any Q-point.

Add the dipole-dipole part for that Q-point

)(~', qC kk!

βα

∗αβ,kZ

∞v,γε

)(~', qCnakk!

βα

Fourier Transform to obtain CSRkα ,k 'β (0,b)

Enforce sum-rules asr

dipdip

ifcflag

Abinit

Anaddb

chneut

Diagonalize dynamical matrix to find phonon frequencies

Time-consuming


Phonon dispersion curves of ZrO2 High - temperature : Fluorite structure ( , one formula unit per cell ) Fm3m

Supercell calculation + interpolation ! Long-range dipole-dipole

interaction not taken into account

ZrO2 in the cubic structure at the equilibrium lattice constant a0 = 5.13 Å.

DFPT (Linear-response) with = 5.75

= -2.86 = 5.75

LO - TO splitting 11.99 THz Non-polar mode is OK

ZZr*

Z0*

ε∞

Wrong behaviour

(From Detraux F., Ghosez Ph. and Gonze X., Phys. Rev. Lett. 81, 3297 (1998) - Comment to the Parlinski & al paper)

(From Parlinski K., Li Z.Q., and Kawazoe Y., Phys. Rev. Lett. 78, 4063 (1997))


Phonon dispersion relations. (a)  Ideal cubic phase : unstable.

(b) Condensations of the unstable phonon modes generate a (meta) stable orthorhombic phase

MgSiO3 CUBIC (5at/cell)

ORTHORHOMBIC (20at/cell)

Analysis of instabilities


Thermodynamic properties from DFPT


Statistical physics : phonons = bosons Harmonic approximation : phonons are independent particles, obeying Bose-Einstein statistics

Internal energy

Uphon = !ω

0

ωmax

∫ n(ω ) + 12

⎛⎝⎜

⎞⎠⎟g(ω )dω

Energy of the harmonic oscillator Phonon density of states

All vibrational contributions to thermodynamic properties, in the harmonic approximation, can be calculated in this manner.

1

1)(−

=TkBe

n ωω


Phonon density of states For each frequency channel, count the “number” of phonon modes m = index of pattern of vibration, = a crystalline momentum (=> velocity of the vibrational wave)

(cm

) (cm-1)

-quartz

stishovite

α

!q

)(31)( ∑ −=

qmqm

atnorm Nng ωωδω


Helmoltz free energy and specific heat

quartz

stishovite Vibrational contribution to F :

Vibrational contribution to Cv :

TSUF −=

VVVV T

FTTST

TUC ⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂−=⎟

⎠⎞⎜

⎝⎛∂∂=⎟

⎠⎞⎜

⎝⎛∂∂= 2

2

ωωωω

dgTk

TkNnFB

Bat )(2

sinh2ln3max

0∫

⎭⎬⎫

⎩⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛=Δ

ωωωωω

dgTkTk

kNnCBB

BatV ∫ ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

max

0

2

2

)(2

csch2

3


Tn

BVT mq

mqmq mq ∂

∂= ∑

)(13

)( ,,

, ,

!

!

! !"

ωγ

ωα

)(ln)(ln ,

, Vqm

qm ∂∂

−=!

!

ωγ

Ab initio thermal expansion

Alternative path : minimisation of free energy


The thermal expansion contribution

Ab initio thermal expansion

Linear thermal expansion coefficient of bulk silicon

G.-M. Rignanese, J.-P. Michenaud and XG Phys. Rev. B 53, 4488 (1996)


Phonons : LDA ? GGA ?


... Lattice parameters from LDA are usually underestimated

... GGA exists in many different flavors (e.g. PBE, PBEsol, AM05, ...), PBE tends to overestimate, PBEsol is better, etc ...

Effect of the choice of XC flavor on phonon frequencies, dielectric tensor, Born effective charges ?

Exhaustive study : L. He et al, Phys. Rev. B89, 064305 (2014) Studied (cf LibXC) :

LDA, PBE, PBEsol, AM05, WC, HTBS for Si, quartz, stishovite, zircon, periclase (MgO), copper

Message : in general, at relaxed atomic parameters, LDA performs better ...

DFPT : use it with LDA ? GGA-PBE ... ?



Gamma phonons of zircon

L. He et al, Phys. Rev. B89, 064305 (2014)



Thermal expansion and T-dependent bulk modulus of copper

L. He et al, Phys. Rev. B89, 064305 (2014)


Phonons in weakly bonded systems


For the last decade : interest in layered and other nanostructured materials. Graphene, transition metal dichalcogenides, etc … - Interesting transport properties - Topological materials - Li or Na insertion in layered materials

Layered materials


Local Density Approximation and Generalized Gradient Approximation only rely on local density, gradients, etc …

Weak bonding : LDA ? GGA ? Beyond ?

Exc n[ ] = n(r1)ε xc (r1;n)∫ dr1

Van der waals : intrinsically non-local, long range electron-electron correlation !New (classes of) functionals DFT-vDW-DF ; DFT-vDW-WF ; DFT-D2, -D3, -D3(BJ) ; …

ExcGGA n[ ] = n(r1)εxc

GGA(n(r1), ∇n(r1) )∫ dr1

ExcLDA n[ ] = n(r1)εxc

LDA(n(r1))∫ dr1


DFT+D3(BJ)

Interlayer parameter d (nm) GGA(PBE) 0.44 +D3(BJ) 0.337 Exp. 0.334

Primitive cell volume (nm^3) [Pbca - 4 Benzene rings] GGA(PBE) >0.600 +D3(BJ) 0.455 Exp. 0.4625


Phonons in benzene crystal Phonons at Gamma


Summary -  Phonon eigenmodes and frequencies: solutions of eigenproblem from dynamical matrices -  Density-Functional Perturbation Theory : ideal for accurate computation of dynamical matrices -  Interatomic force constants for polar insulators: long ranged due to dipole-dipole interaction. -  Fourier interpolation + treatment of dipole-dipole interaction = effective interpolation of dynamical matrices => phonon band structures. - Phonon band structures easily computed for insulators, metals, ... -Thermodynamics (specific heat, thermal expansion ...) - New functionals : OK for DFPT in weakly bonded systems

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Density-functional Perturbation Theory